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Effects of disorder on the in-plane density of states of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ are investigated by planar tunneling spectroscopy. Low-fluence 1-MeV ${\mathrm{He}}^{+}$ irradiation reduces the ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ ${T}_{c}$ and mean-free path without altering the junction quality. As the dirty limit is approached, the zero-bias conductance peak (ZBCP) disappears, as expected for a quasiparticle Andreev bound state. Thus, the quasiparticle scattering rate may be determined by the ZBCP magnitude. This rate is a direct measure of the pair-breaking strength and accounts for the ${T}_{c}$ suppression.