  Structural transformations, electronic spectra and ballistic transport in
pulled gold nanowires are investigated with ab initio simulations, and
correlated with recent measurements. Strain-induced yield of an initial
double-strand wire results first in formation of a bent-chain which transforms
upon further elongation to a linear atomic chain exhibiting dimerized atomic
configurations. These structures are stabilized by directional local bonding
with spd-hybridization. The conductance of the initial double-stranded contact
is close to 2g_0 and it drops sharply to 1g_0 during the transformation to a
single chain, exhibiting subsequently a 1g_0 plateau extending over an
elongation well above typical Au-Au distances.
