  We study Andreev reflection in a one-dimensional square-well pair-potential.
We discuss the history of the model. The current-phase relation is presented as
a sum over Matsubara frequencies. How the current arises from bound and
continuum levels is found by analytic continuation. We discuss two limiting
cases of the square-well potential, the zero-length well and the infinite well.
The model is quantitatively valid in some cases but forms the basis for
understanding a wide range of problems in inhomogeneous superconductivity and
superfluidity.
