  We construct a model to study the localization properties of nanowires of
dopants in silicon (Si) fabricated by precise ionic implantation or STM
lithography. Experiments have shown that Ohm's law holds in some cases, in
apparent defiance to the Anderson localization theory in one dimension. We
investigate how valley interference affects the traditional theory of
electronic structure of disordered systems. Each isolated donor orbital is
realistically described by multi-valley effective mass theory (MV-EMT). We
extend this model to describe chains of donors as a linear combination of
dopant orbitals. Disorder in donor positioning is taken into account, leading
to an intricate disorder distribution of hoppings between nearest neighbor
donor sites (donor-donor tunnel coupling) -- an effect of valley interference.
The localization length is obtained for phosphorous (P) donor chains from a
transfer matrix approach and is further compared with the chain length. We
quantitatively determine the impact of uncertainties $\delta R$ in the
implantation position relative to a target and also compare our results with
those obtained without valley interference. We analyse systematically the aimed
inter-donor separation dependence ($R_0$) and show that fairly diluted donor
chains ($R_0=7.7$ nm) may be as long as 100 nm before the effective onset of
Anderson localization, as long as the positioning error is under a lattice
parameter ($\delta R <0.543$ nm).
