  We report our observation of the training effect on dc electrical properties
in a nanochain of BiFeO$_3$ as a result of large scale migration of defects
under combined influence of electric field and Joule heating. We show that an
optimum number of cycles of electric field within the range zero to $\sim$1.0
MV/cm across a temperature range 80-300 K helps in reaching the stable state
via a glass-transition-like process in the defect structure. Further treatment
does not give rise to any substantial modification. We conclude that such a
training effect is ubiquitous in pristine nanowires or chains of oxides and
needs to be addressed for applications in nanoelectronic devices.
