  Andreev-reflection spectroscopy of elemental superconductors in contact with
non-magnetic normal metals reveals that the strength of normal-reflection
varies only slightly. This observation imposes strong constrictions on the
three possible normal-reflection mechanisms: tunneling through a dielectric
barrier, reflection due to the different electronic properties of the two
electrodes, and diffusive transport caused by elastic scattering in the contact
region. We discuss in detail the role played by Fermi-surface mismatch,
represented by the different Fermi velocities on both sides of the contact
interface. We find that it is at least not the dominant mechanism and possibly
completely absent in the Andreev-reflection process.
