In a conductor, the current depends on the the number of carriers(n), charge of the carriers(e), area of the conductor(A) and the drift velocity(v_d). At a fixed temperature, n,e,A are fixed while v_d is directly proportional to the electric field(E). As electrons move towards junction, they face an additional electric field due to the accumulation of charges at the surface of the wire. This additional electric field alters the momentum of charges and thus they are not conserved.