In the quest of nano-objects for future electronics, silicon nanowires could possibly take over carbon nanotubes. Here we show the growth by self-organization of straight, massively parallel silicon nanowires having a width of 1.6 nm, which are atomically perfect and highly metallic conductors. Surprisingly, these silicon nanowires display a strong symmetry breaking across their widths with two chiral species that self-assemble in large left-handed and right-handed magnetic-like domains.