  To investigate the validity of the Wiedemann-Franz (WF) law in disordered but
metallic cuprates, the low-temperature charge and heat transport properties are
carefully studied for a series of impurity-substituted and carrier-overdoped
La_{1.8}Sr_{0.2}Cu_{1-z}M_zO_4 (M = Zn or Mg) single crystals. With moderate
impurity substitution concentrations of z = 0.049 and 0.082 (M = Zn), the
resistivity shows a clear metallic behavior at low temperature and the WF law
is confirmed to be valid. With increasing impurity concentration to z = 0.13 (M
= Zn) or 0.15 (M = Mg), the resistivity shows a low-T upturn but its
temperature dependence indicates a finite conductivity in the T \to 0 limit. In
this weakly-localized metallic state that is intentionally achieved in the
overdoped regime, a {\it negative} departure from the WF law is found, which is
opposite to the theoretical expectation.
