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Bandstructure effects in p-channel MOS (PMOS) transport of strongly quantized silicon nanowire FETs in various transport orientations are examined. A 20-band sp3d5s* spin-orbit (SO) coupled atomistic tight-binding model coupled to a self-consistent Poisson solver is used for the valence-band dispersion calculation. A ballistic FET model is used to evaluate the capacitance and current-voltage characteristics. The dispersion shapes and curvatures are strong functions of device size, lattice orientation, and bias, and cannot be described within the effective mass approximation. The anisotropy of the confinement mass in the different quantization directions can cause the charge to preferably accumulate in the (110) and then on the (112) rather than on (100) surfaces, leading to significant differences in the charge distributions for different wire orientations. The total gate capacitance of the nanowire FET devices is, however, very similar for all wires in all the investigated transport orientations (, , ), and is degraded from the oxide capacitance by ~30%. The  and then the  oriented nanowires indicate highest carrier velocities and better on-current performance compared to  wires. The dispersion features and quantization behavior, although a complicated function of physical and electrostatic confinement, can be explained at first order by looking at the anisotropic shape of the heavy-hole valence band.