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We have investigated the scaling properties of  InAs nanowire MOSFETs in the ballistic limit. The nanowire band structure has been calculated with an sp3d5 s* tight-binding model for nanowire diameters between 2 and 25 nm. Both the effective band gap and the effective masses increase with confinement. Using the atomistic dispersion relations, the ballistic currents and corresponding capacitances have been calculated with a semianalytical model. It is shown that the InAs nanowire MOSFET with diameters scaled below 15-20 nm can be expected to operate close to the quantum capacitance limit, assuming a high-kappa dielectric thickness of 1-1.5 nm. We have also investigated the evolution of ft and the gate delay, both showing improvements as the device is scaled. The very small intrinsic gate capacitance in the quantum limit makes the device susceptible to parasitic capacitances.