  We study the transport properties of graphene nanoribbons of standardized 30
nm width and varying lengths. We find that the extent of the gap observed in
transport as a function of Fermi energy in these ribbons (the "transport gap")
does not have a strong dependence on ribbon length, while the extent of the gap
as a function of source-drain voltage (the "source-drain gap") increases with
increasing ribbon length. We anneal the ribbons to reduce the amplitude of the
disorder potential, and find that the transport gap both shrinks and moves
closer to zero gate voltage. In contrast, annealing does not systematically
affect the source-drain gap. We conclude that the transport gap reflects the
overall strength of the background disorder potential, while the source-drain
gap is sensitively dependent on its details. Our results support the model that
transport in graphene nanoribbons occurs through quantum dots forming along the
ribbon due to a disorder potential induced by charged impurities.
