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A comparison of nanowire FETs (NWFETs) of identical geometries but operating in two different regimes, namely, the quantum capacitance (QC) and classical capacitance (CC) regimes, is presented. n-type InSb and InAs NWFETs up to ~50 nm in diameter operate in the QC limit (QCL), and the corresponding p-type NWFETs operate in the CC limit. Drive currents at a fixed gate overdrive for the n- and p-type devices are found to be well matched. Nevertheless, the p-type devices have twice the delay times, half the intrinsic cutoff frequencies, twice the power-delay products, and four to five times the energy-delay products of the n-type devices, assuming transport is ballistic. Analytical expressions are derived for the QC, the current, the charge, the power-delay product, the energy-delay product, the gate delay time, and the cutoff frequency for a single-moded device operating in the QCL. The expressions for the power-delay product, energy-delay product, and the cutoff frequency are fundamental limits for such devices.