  We combine the ideas of scaling theory and universal conductance fluctuations
with density-functional theory to analyze the conductance properties of doped
silicon nanowires. Specifically, we study the cross-over from ballistic to
diffusive transport in boron (B) or phosphorus (P) doped Si-nanowires by
computing the mean free path, sample averaged conductance <G>, and
sample-to-sample variations std(G) as a function of energy, doping density,
wire length, and the radial dopant profile. Our main findings are: (i) the main
trends can be predicted quantitatively based on the scattering properties of
single dopants; (ii) the sample-to-sample fluctuations depend on energy but not
on doping density, thereby displaying a degree of universality, and (iii) in
the diffusive regime the analytical predictions of the DMPK theory are in good
agreement with our ab initio calculations.

Mesoscale and Nanoscale Physics