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In this paper, we present a self-consistent and 3D quantum simulator for Si-nanowire transistors based on the Wigner function model and multidimensional Schrodinger-Poisson algorithm. To achieve a sufficient numerical accuracy for calculating subthreshold current, we introduced a third-order differencing scheme for discretizing the drift term in the Wigner transport equation. By comparing with semiclassical Boltzmann and nonequilibrium Green's function approaches, the validity of the present simulator is confirmed. We also demonstrate that the source-drain tunneling is a critical physical phenomenon related to a scaling limit of nanowire devices, and the semiclassical simulation measurably underestimates a minimum gate length.