dc-current transport and ac Josephson effect in quantum junctions at low voltage

Multiple Andreev scattering in single-mode superconducting junctions with arbitrary normal electron transparency 01 is studied in the limit of low applied voltage eV\ensuremath{\ll}\ensuremath{\Delta}. A quasiclassical approach is developed for investigation of the dense lattice of inelastic sidebands associated with multiple Andreev scattering, which gives a global description of inelastic-scattering amplitudes and spectral distribution of the current. The crossover from the contact to the tunnel regime is investigated for the dc current and ac Josephson current as function of junction transparency and applied voltage. A mesoscopic interference effect in junctions with intermediate transparency is discussed. This effect shows up in oscillating features of the current of thermal excitations.