  We have investigated theoretically the conductance of a Normal-Superconductor
point-contact in the tunnel limit and analyzed the quantum interference effects
originating from the scattering of quasiparticles by point-like defects.
Analytical expressions for the oscillatory dependence of the conductance on the
position of the defect are obtained for the defect situated either in the
normal metal, or in the superconductor. It is found that the amplitude of
oscillations significantly increases when the applied bias approaches the gap
energy of the superconductor. The spatial distribution of the order parameter
near the surface in the presence of a defect is also obtained.
