The recent observation of ultralow resistivity in highly doped, atomic-scale silicon wires has sparked interest in what limits conduction in these quasi-1D systems. Here we present electron transport measurements of gated Si∶P wires of widths 4.6 and 1.5 nm. At 4.6 nm we find an electron mobility, μel ≃ 60 cm2/Vs, in excellent agreement with that of macroscopic Hall bars. Metallic conduction persists to millikelvin temperatures where we observe Gaussian conductance fluctuations of order δG ∼ e2/h. In thinner wires (1.5 nm), metallic conduction breaks down at G ≲ e2/h, where localization of carriers leads to Coulomb blockade. Metallic behavior is explained by the large carrier densities in Si∶P δ-doped systems,allowing the occupation of all six valleys of the silicon conduction band, enhancing the number of 1D channels and hence the localization length.