  Experiments on disordered superconducting thin films have revealed a
magnetoresistance peak of several orders of magnitude. Starting from the
disordered negative-U Hubbard model, we employ an ab initio approach that
includes thermal fluctuations to calculate the resistance, and fully reproduces
the experimental phenomenology. Maps of the microscopic current flow and local
potential allow us to pinpoint the source of the magnetoresistance peak -- the
conducting weak links change from normal on the low-field side of the peak to
superconducting on the high-field side. Finally, we formulate a simple
one-dimensional model to demonstrate how small superconducting regions will act
as weak links in such a disordered thin film.
