  We examine transport properties of superconducting hybrid mesoscopic
structures, in both the diffusive and ballistic regimes. For diffusive
structures, analytic results from quasi-classical theory are compared with
predictions from numerical, multiple-scattering calculations performed on small
structures. For all structures, the two methods yield comparable results and in
some cases, quantitative agreement is obtained. These results not only
demonstrate that quasi-classical theory can yield the ensemble averaged
conductance $<G>$ of small structures of dimensions of order 10- 20 Fermi
wavelengths, but also establish that numerical scattering calculations on such
small structures can yield results for $<G>$ which are characteristic of much
larger systems. Having compared the two approaches, we extend the
multiple-scattering analysis to the ballistic limit, where the sample
dimensions become smaller than the elastic mean free path and demonstrate that
the properties of certain Andreev interferometers are unchanged in the clean
limit.
