  The Autocorrelation function of spectral determinants (ASD) is used to probe
the sensitivity of a two-dimensional disordered electron gas to the system's
size L.
  For weak magnetic fields ASD is shown to depend only trivially on L, which is
a strong indication that all states are localized.
  From nontrivial dependence of ASD on L for infinite L at a Hall conductance
of 1/2 e^2/h we deduce the existence of critical wave functions at this point,
as long as the disorder strength does not exceed a critical value.
