  Quantum threshold reflection is a well known quantum phenomenon which
prescribes that at threshold, except for special circumstances, a quantum
particle scattering from any potential, even if attractive at long range, will
be reflected with unit probability. In the past, this property has been
associated with the so-called badlands region of the potential, where the
semiclassical description of the scattering fails due to a rapid spatial
variation of the deBroglie wavelength. This badlands region occurs far from the
strong interaction region of the potential and has therefore been used to
"explain" the quantum reflection phenomenon. In this paper, we show that the
badlands region of the interaction potential is immaterial. The extremely long
wavelength of the scattered particle at threshold is much longer than the
spatial extension of the badlands region which therefore does not affect the
scattering. For this purpose, we review the general proof for the existence of
quantum threshold reflection to stress that it is only a consequence of
continuity and boundary conditions. The nonlocal character of the scattering
implies that the whole interaction potential is involved in the phenomenon. We
then provide a detailed numerical study of the threshold scattering of a
particle by a Morse potential especially in the time domain. We compare exact
quantum computations with incoherent results obtained from a classical Wigner
approximation. This study shows that close to threshold the time dependent
amplitude of the scattered particle is negligible in the badlands region and
that the mean flight time of the particle is not shortened due to a local
reflection from the badlands region. This study should serve to definitely rule
out the badlands region as a qualitative guide to the properties of quantum
threshold reflection.
