  Temperature dependent weak localization is measured in metallic nanowires in
a previously unexplored size regime down to width $w=5$ nm. The dephasing time,
$\tau_{\phi}$, shows a low temperature $T$ dependence close to quasi-1D
theoretical expectations ($\tau_{\phi} \sim T^{-2/3}$) in the narrowest wires,
but exhibits a relative saturation as $T \to 0$ for wide samples of the same
material, as observed previously. As only sample geometry is varied to exhibit
both suppression and divergence of $\tau_{\phi}$, this finding provides a new
constraint on models of dephasing phenomena.
