The proximity effect and subgap conductivity in superconductor-barrier-normal metal contacts

On the basis of microscopic equations for matrix Green's functions with boundary conditions at the interface, we calculate the conductance of S/N contacts of different types (a planar S/N junction and a one-dimensional S/N channel) as a function of bias voltage. It is shown that even at zero temperature the subgap conductance S(V) (V<Δ) may be significant (of the order of the contact conductance in the normal state). The subgap conductance is caused by a term in the current which leads to the interference current at TTc in SIS junctions. The form of S(V) depends essentially on the relation between the barrier transmittance and the depairing rate in the N region. ((orig.))