  We present measurements of the magnetoconductance of long and narrow quasi
one-dimensional gold wires containing magnetic iron impurities in a temperature
range extending from $15 $mK to $4.2 $K. The dephasing rate extracted from the
weak antilocalisation shows a pronounced plateau in a temperature region of
$300 $mK - $800 $mK, associated with the phase breaking due to the Kondo
effect. Below the Kondo temperature the dephasing rate decreases linearly with
temperature, in contradiction with standard Fermi-liquid theory. Our data
suggest that the formation of a spin glass due to the interactions between the
magnetic moments are responsible for the observed anomalous temperature
dependence.
