Subgap conductivity of a superconductor-normal-metal tunnel interface

At low temperatures, the transport through a superconductor--normal-metal tunnel interface is due to tunneling of electrons in pairs. We show that the rate for this process is often determined by the interference of the electron waves on a space scale determined by the coherence length. Therefore, the subgap current strongly depends on the layout of the electrodes within this space scale. The approach developed allows us to evaluate the subgap current for different layouts of interest.