  We describe electrical transport in ideal single-layer graphene at zero
applied bias. There is a crossover from collisionless transport at frequencies
larger than k_B T/hbar (T is the temperature) to collision-dominated transport
at lower frequencies. The d.c. conductivity is computed by the solution of a
quantum Boltzmann equation. Due to a logarithmic singularity in the collinear
scattering amplitude (a consequence of relativistic dispersion in two
dimensions) quasi-particles and -holes moving in the same direction tend to an
effective equilibrium distribution whose parameters depend on the direction of
motion. This property allows us to find the non-equilibrium distribution
functions and the quantum critical conductivity exactly to leading order in
1/|ln(alpha)| where alpha is the coupling constant characterizing the Coulomb
interactions.
