  We present a detailed study of the high-current transport properties of
graphene devices patterned in a four-point configuration. The current tends to
saturate as the voltage across graphene is increased but never reaches the
complete saturation as in metallic nanotubes. Measurements are compared to a
model based on the Boltzmann equation, which includes electron scattering
processes due to charged and neutral impurities, and graphene optical-phonons.
The saturation is incomplete because of the competition between disorder and
optical-phonon scattering.
