  We show that a coherent picture of the dc conductivity of monolayer and
bilayer graphene at finite electronic densities emerges upon considering that
strong short-range potentials are the main source of scattering in these two
systems. The origin of the strong short-range potentials may lie in adsorbed
hydrocarbons at the surface of graphene. The equivalence among results based on
the partial-wave description of scattering, the Lippmann-Schwinger equation,
and the T-matrix approach is established. Scattering due to resonant impurities
close to the neutrality point is investigated via a numerical computation of
the Kubo formula using a kernel polynomial method. We find that relevant
adsorbate species originate impurity bands in monolayer and bilayer graphene
close to the Dirac point. In the midgap region, a plateau of minimum
conductivity of about $e^2/h$ (per layer) is induced by the resonant disorder.
In bilayer graphene, a large adsorbate concentration can develop an energy gap
between midgap and high-energy states. As a consequence, the conductivity
plateau is supressed near the edges and a "conductivity gap" takes place.
Finally, a scattering formalism for electrons in biased bilayer graphene,
taking into account the degeneracy of the spectrum, is developed and the dc
conductivity of that system is studied.
