  We have systematically investigated the low-temperature electron dephasing
times $\tau_\phi$ in more than 40 three-dimensional polycrystalline impure
metals with distinct material characteristics. In all cases, a saturation of
the dephasing time is observed below about a (few) degree(s) Kelvin, depending
on samples. The value of the saturated dephasing time $\tau_0$ [$\equiv
\tau_\phi (T \to 0 {\rm K})$] falls basically in the range 0.005 to 0.5 ns for
all samples. Particularly, we find that $\tau_0$ scales with the electron
diffusion constant $D$ as $\tau_0 \sim D^{- \alpha}$, with $\alpha$ close to or
slightly larger than 1, for over two decades of $D$ from about 0.1 to 10
cm$^2$/s. Our observation suggests that the saturation behavior of $\tau_\phi$
is universal and intrinsic in three-dimensional polycrystalline impure metals.
A complete theoretical explanation is not yet available.
