  Transport properties of a two-band system with spectral nodes are studied in
the presence of random scattering. Starting from a Grassmann functional
integral, we derive a bosonic representation that is based on random phase
fluctuations. Averaging leads to a graphical representation of the correlation
function with entangled random walks and 4-vertices. In the strong scattering
limit we derive a complex transition amplitude. For the example of
two-dimensional Dirac fermions we obtain a localization length proportional to
the inverse scattering rate.
