  We study a weakly disordered 2D electron gas with two bands and a spectral
node within the weak-localization approach and compare its results with those
of Gaussian fluctuations around the self-consistent Born approximation. The
appearance of diffusive modes depends on the type of disorder. In particular,
we find for a random gap a diffusive mode only from ladder contributions,
whereas for a random scalar potential the diffusive mode is created by ladder
and by maximally crossed contributions. The ladder (maximally crossed)
contributions correspond to fermionic (bosonic) Gaussian fluctuations. We
calculate the conductivity corrections from the density--density Kubo formula
and find a good agreement with the experimentally observed V-shape conductivity
of graphene.
