We calculate the transport characteristics of thin silicon nanowires (NWs) up to 12 nm in diameter. The sp(3)d(5)s*-spin-orbit-coupled atomistic tight-binding (TB) model is used for the electronic structure calculation. Linearized Boltzmann transport theory is applied for transport, including carrier scattering by phonons and surface roughness (SRS). We show that for certain transport orientations, confinement in specific directions enhances the curvature of the subbands such that large improvements can be achieved in carrier velocity and mobility. This is observed for p-type [1 1 0] and [1 1 1] NWs for which confinementinduced bandstructure changes improve the phonon limited mobility by >2X compared to bulk values. Such improvement suggests very large carrier mean-free-paths for scattering and is large enough to compensate for the detrimental effect of SRS. This effect is observed at a smaller degree for n-type [1 1 0] NWs as well. The bandstructure engineering techniques we describe originate from the warped, anisotropic bulk bands, and can be also applied to 20 ultra-thin-body (UTB) layers as well as other channel materials. Our results may provide understanding of recent experimental observations, as well as guidance in the design of NW and UTB channel devices with improved transport properties. (C) 2011 Elsevier Ltd. All rights reserved.