  We investigate the system of a particle moving on a half line x >= 0 under
the general walls at x = 0 that are permitted quantum mechanically. These
quantum walls, characterized by a parameter L, are shown to be realized as a
limit of regularized potentials. We then study the classical aspects of the
quantum walls, by seeking a classical counterpart which admits the same time
delay in scattering with the quantum wall, and also by examining the
WKB-exactness of the transition kernel based on the regularized potentials. It
is shown that no classical counterpart exists for walls with L < 0, and that
the WKB-exactness can hold only for L = 0 and L = infinity.
