  The field-effect mobility of graphene devices is discussed. We argue that the
graphene ballistic mean free path 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 300nm at room-temperature for
a carrier concentration of ~1e12/cm2 and that a substantial series resistance
of around 300ohmum has to be taken into account. Furthermore, we demonstrate
first quantum capacitance measurements on single-layer graphene devices.
