  We investigate the density and temperature-dependent conductance of graphene
nanoribbons with varying aspect ratio. Transport is dominated by a chain of
quantum dots forming spontaneously due to disorder. Depending on ribbon length,
electron density, and temperature, single or multiple quan- tum dots dominate
the conductance. Between conductance resonances cotunneling transport at the
lowest temperatures turns into activated transport at higher temperatures. The
density-dependent activation energy resembles the Coulomb gap in a quantitative
manner. Individual resonances show signatures of multi-level transport in some
regimes, and stochastic Coulomb blockade in others.
