  Transparency of the metal/superconductor interface is discussed in a frame of
the Kupryanov-Lukichev (K-L) quasiclassical boundary conditions. It is shown
that the original K-L boundary transparency is expressed via the single-channel
Landauer conductance. It is a source of dramatic discrepancy between the theory
and experiments on the superconducting proximity effect. The improved
multi-channel derivation is proposed for the boundary conditions, which
rehabilitates consistency of the quasiclassical approach to the boundary
transparency problem, and eliminates at least an order of magnitude of
discrepancy between the theory and the experiment. The influence of the
interface roughness and wave function symmetry mismatch at the boundary on the
interface transparency is discussed. Warning is made to theories that use the
quasiclassical boundary conditions with the interface transparencies close to
unity.
