  Boltzmann transport theory fails near the linear band-crossing of
single-layer graphene and near the quadratic band-crossing of bilayer graphene.
We report on a numerical study which assesses the role of inter-band coherence
in transport when the Fermi level lies near the band-crossing energy of bilayer
graphene. We find that interband coherence enhances conduction, and that it
plays an essential role in graphene's minimum conductivity phenomena. This
behavior is qualitatively captured by an approximate theory which treats
inter-band coherence in a relaxation-time approximation. On the basis of this
short-range-disorder model study, we conclude that electron-hole puddle
formation is not a necessary condition for finite conductivity in graphene at
zero average carrier density.
