  We have investigated the disorder of epitaxial graphene close to the charge
neutrality point (CNP) by various methods: i) at room temperature, by analyzing
the dependence of the resistivity on the Hall coefficient ; ii) by fitting the
temperature dependence of the Hall coefficient down to liquid helium
temperature; iii) by fitting the magnetoresistances at low temperature. All
methods converge to give a disorder amplitude of $(20 \pm 10)$ meV. Because of
this relatively low disorder, close to the CNP, at low temperature, the sample
resistivity does not exhibit the standard value $\simeq h/4e^2$ but diverges.
Moreover, the magnetoresistance curves have a unique ambipolar behavior, which
has been systematically observed for all studied samples. This is a signature
of both asymmetry in the density of states and in-plane charge transfer. The
microscopic origin of this behavior cannot be unambiguously determined.
However, we propose a model in which the SiC substrate steps qualitatively
explain the ambipolar behavior.
