  The subgap conductivity of a normal-superconductor (NS) tunnel junction is
thought to be due to tunneling of two electrons. There is a strong interference
between these two electrons, originating from the spatial phase coherence in
the normal metal at a mesoscopic length scale and the intrinsic coherence of
the superconductor.
  We evaluated the interference effect on the transport through an NS junction.
  We propose the layouts to observe drastic Aharonov-Bohm and Josephson
effects.
