  We highlight a new "transverse" interference effect, arising from the
coupling between electrons and holes, induced by a superconducting island in
contact with a normal metal. As an example we compute the electrical
conductance G of a T-shaped mesoscopic sample, formed by joining a horizontal
bar of normal metal to a vertical leg of the same material. With a
superconducting island located on the vertical leg and the current flowing
horizontally, we examine the dependence of G on the distance L of the island
from the horizontal bar.
  Of particular interest is the differential conductance G(E) at an applied
voltage E=eV, which due to quantum interference between electrons and Andreev
reflected holes, is predicted to oscillate with both L and E. For a system with
a spherical Fermi surface, the period of oscillation with E is 3.142 (E_F/k_FL)
and with L is 3.142 (E_F/k_F E), where E_F is the Fermi energy and k_F the
Fermi wavevector.
