Is the motion of a charge across a junction momentum conserving? Why or why not?

(N/A) The motion of a charge across a junction is generally not momentum-conserving.
When a charge carrier (such as an electron) moves through a conductor,it experiences collisions with the lattice ions,which exert external forces on the charge.
According to Newton's second law,the rate of change of momentum of a system is equal to the net external force acting on it.
Since the lattice ions exert a force on the charge carriers to maintain a steady drift velocity $v_{d} = \frac{eE\tau}{m}$,the momentum of the charge carrier is not conserved because the external force from the lattice is non-zero.
Furthermore,at a junction,the redistribution of charge creates localized electric fields that further alter the momentum of the charge carriers.