  In classical external gauge fields that fall off less fast than the inverse
of the evolution parameter (time) of the system the implementability of a
unitary perturbative scattering operator ($S$-matrix) is not guaranteed,
although the field goes to zero. The importance of this point is exposed for
the counter-example of low-dimensionally expanding systems. The issues of gauge
invariance and of the interpretation of the evolution at intermediate times are
also intricately linked to that point.
