Cancellation of quasiparticle mass enhancement in the conductance of point contacts

The high fraction of Andreev reflections that has been observed in noble-metal point contacts with the high-${\mathit{T}}_{\mathit{c}}$ oxides ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ and ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Sr}}_{\mathit{x}}$ ${\mathrm{CuO}}_4$ as well as with the heavy-fermion superconductors ${\mathrm{UPt}}_3$ and ${\mathrm{URu}}_2$ ${\mathrm{Si}}_2$ suggests that the boundary condition at the interface involves Fermi velocities without mass-enhancement factors. We show theoretically that this is indeed the case for a self-energy Σ(ω,k) only weakly dependent on k. A comparison between the velocities derived from point-contact experiments with known values of the fully dressed quasiparticle velocities allows a determination of the mass-enhancement factor.