  We consider the evolution of quantum fermi fields in Minkowski gauge field
backgrounds. Our motivation is the study of anomalous fermion number violating
processes. We derive selection rules for fermion scattering amplitudes which
relate the violation of fermionic charge to the change in winding between early
and late times of the vacuum part of the gauge field. We find that the amount
of fermion number violation is always integer, even when the topological charge
of the gauge field is fractional. As an explicit example, we apply our results
to spherically symmetric Minkowski solutions.
