  We show that the voltage drop of specially prepared
normal-superconducting-normal nanostructures show quantum Andreev oscillations
as a function of magnetic field or input current. These oscillations are due to
the interference of the electron wave function between the normal parts of the
structure that act as reflective interfaces, i.e. our devices behave as a
Fabry-Perot interferometer for conduction electrons. The observed oscillations
and field periods are well explained by theory.
