  At low temperatures, the transport through a superconducting-normal tunnel
interface is due to tunneling of electrons in pairs. The probability for this
process is shown to depend on the layout of the electrodes near the tunnel
junction, rather than on properties of the tunnel barrier. This dependence is
due to interference of the electron waves on a space scale determined by the
coherence length, either in the normal or the superconducting metal. The
approach developed allows us to evaluate the subgap current for different
layouts of interest.
