We study electron transport properties of a monoatomic graphite layer
(graphene) with different types of disorder. We show that the transport
properties of the system depend strongly on the character of disorder. Away
from half filling, the concentration dependence of conductivity is linear in
the case of strong scatterers, in line with recent experimental observations,
and logarithmic for weak scatterers. At half filling the conductivity is of the
order of e^2/h if the randomness preserves one of the chiral symmetries of the
clean Hamiltonian; otherwise, the conductivity is strongly affected by
localization effects.Comment: 21 pages, 9 figure