  Graphene, the first true two-dimensional material still reveals the most
remarkable transport properties among the growing class of two-dimensional
materials. Although many studies have investigated fundamental scattering
processes, the surprisingly large variation in the experimentally determined
resistances associated with a localized defect is still an open issue. Here, we
quantitatively investigate the local transport properties of graphene prepared
by polymer assisted sublimation growth (PASG) using scanning tunneling
potentiometry. PASG graphene is characterized by a spatially homogeneous
current density, which allows to analyze variations in the local
electrochemical potential with high precision. We utilize this possibility by
examining the local sheet resistance finding a significant variation of up to
270% at low temperatures. We identify a correlation of the sheet resistance
with the stacking sequence of the 6H-SiC substrate as well as with the distance
between the graphene sheet and the substrate. Our results experimentally
quantify the strong impact of the graphene-substrate interaction on the local
transport properties of graphene.
