  Electrons can be reflected at an interface between two metals because of a
dielectric barrier or different properties of the Fermi surface. Andreev
reflection allows to directly measure normal reflection when one of the metals
is a superconductor. We have investigated normal reflection at interfaces
between non-magnetic normal metals and superconducting Nb (Tc = 9.2 K) and Al
(Tc = 1.2 K). The distribution of the values of the relative strength of the
interface barrier, Z, for a number of contacts of a specific metal combination
shows a well-defined peak which can be attributed to Fermi surface mismatch.
Our reflection coefficients are generally smaller than those predicted
theoretically or those derived from proximity-effect studies of
normal-superconductor bi-layers.
