  We report resistivity measurements in underdoped YBa$_{2}$Cu$_{3}$O$_{6.6}
$and overdoped Tl$_{2}$Ba$_{2}$CuO$_{6+x}$ single crystals in which the
concentration of defects in the CuO$_{2}$ planes is controlled by electron
irradiation. Low $T$ upturns of the resistivity are observed in both cases for
large defect content. In the Tl compound the decrease of conductivity scales as
expected from weak localization theory. On the contrary in
YBa$_{2}$Cu$_{3}$O$_{6.6}$ the much larger low T contribution to the
resistivity is proportional to the defect content and might then be associated
to a Kondo like spin flip scattering term. This would be consistent with the
results on the magnetic properties induced by spinless defects.
