  Among the recent discoveries of domain wall functionalities, the observation
of electrical conduction at ferroelectric domain walls in the multiferroic
insulator BiFeO3 has opened exciting new possibilities. Here, we report
evidence of electrical conduction also at 180{\deg} ferroelectric domain walls
in the simpler tetragonal ferroelectric PZT thin films. The observed conduction
shows nonlinear, asymmetric current-voltage characteristics, thermal activation
at high temperatures and high stability. We relate this behavior to the
microscopic structure of the domain walls, allowing local defects segregation,
and the highly asymmetric nature of the electrodes in our local probe
measurements.
