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In a highly doped GaAs semiconductor with superconducting contacts of Al, clear conductance peaks are observed at zero voltage bias and at $V\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\ifmmode\pm\else\textpm\fi{}2\ensuremath{\Delta}/e,$ $\ifmmode\pm\else\textpm\fi{}\ensuremath{\Delta}/e$. The subharmonic energy gap structure originates from Andreev scattering with diffusive, but energy conserving, transport in the GaAs. The zero bias excess conductance is due to phase-coherent transport. Both effects are suppressed when the distance between the superconducting electrodes exceeds the inelastic diffusion length in the GaAs normal channel.

We have measured the low-temperature magnetoresistance of long (>50 mu m), narrow (similar to 1 mu m) quantum wires fabricated from a high-mobility two-dimensional electron gas. In the quantum Hall regime at high magnetic fields we observe that the Shubnikov-de Haas oscillations split into a series of aperiodic sharp peaks. Both the amplitude and sharpness of the peaks are strongly dependent on temperature below 1 K. We attribute the peaks to resonant tunneling between edge states through localized states in the bulk. The results are in quantitative agreement with the theoretical model of Jain and Kivelson.

Weak-localization magnetoresistance has been measured as a function of carrier density using a backgate technique. The phase relaxation rate and the spin-orbit relaxation rate have been determined. The dominating contribution to the phase relaxation rate comes from electron-electron interaction, as demonstrated for samples covering densities from 2.5 to 7.5×10 15 m -2 . Interband scattering is found to enhance phase relaxation. A strictly two-dimensional electron gas (2DEG) quenches the spin-orbit relaxation rate in the plane of the 2DEG, 1/τ so xy , whereas the orthogonal part, 1/τ so z appears added to the phase relaxation rate in the interpretation of the weak-localization magnetoresistance, and can be determined as the saturation value of the phase relaxation at low temperatures

We have extended earlier measurements of the magnetoresistance in a periodically modulated two-dimensional electron gas to high magnetic fields, where the cyclotron radius, r(c), is much smaller than the period, a, of modulation. A giant nonoscillatory magnetoresistance was found to arise beyond the range of low-field (Weiss) oscillations periodic in 1/B. Its value in a magnetic field of a few tesla may exceed the zero-field resistance value, rho-0, by 1 to 2 orders of magnitude. This result is in agreement with the semiclassical theory by Beenakker, if a modulation of the electron mobility is taken into account.

La magnetoresistance est etudiee dans ce gaz d'electrons, aux basses temperatures. Au champ magnetique le plus bas, on observe une localisation faible. On etudie plus particulierement la resistivite aux champs magnetiques ou le produit de la mobilite et du champ magnetique est de l'ordre de l'unite. La correction quantique due a l'interaction electronique conduit neanmoins a une magneto-conductivite quadratique quantifiable