  Weak localization has a strong influence on both the normal and
superconducting properties of metals. In particular, since weak localization
leads to the decoupling of electrons and phonons, the temperature dependence of
resistance (i.e., $\lambda_{tr}$) is decreasing with increasing disorder, as
manifested by Mooij's empirical rule. In addition, Testardi's universal
correlation of $T_{c}$ (i.e., $\lambda$) and the resistance ratio (i.e.,
$\lambda_{tr}$) follows. This understanding provides a new means to probe the
phonon mechanism in superconductors including MgB$_{2}$. The merits of this
method are its applicability to any superconductors and its reliability because
the McMillan's electron-phonon coupling constant $\lambda$ and $\lambda_{tr}$
change in a broad range, from finite values to zero, due to weak localization.
Karkin et al's preliminary data of irradiated MgB$_{2}$ show the Testardi
correlation, indicating that the dominant pairing mechanism in MgB$_{2}$ is the
phonon-mediated interaction.
