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The field-effect mobility of graphene devices is discussed. We argue that the graphene ballistic mean free path, Lball can only be extracted by taking into account both, the electrical characteristics and the channel length dependent mobility. In doing so we find a ballistic mean free path of Lball=300plusmn100 nm at room-temperature for a carrier concentration of ~1012 cm-2 and that a substantial series resistance of around 300 Omega mum has to be taken into account. Furthermore, we demonstrate first quantum capacitance measurements on single-layer graphene devices.