A non-perturbative determination of Z_V and b_V for O(a) improved   quenched and unquenched Wilson fermions
  By considering the local vector current between nucleon states and imposing charge conservation we determine, for $O(a)$ improved Wilson fermions, its renormalisation constant and quark mass improvement coefficient. The computation is performed for both quenched and two flavour unquenched fermions. 
T. Bakeyev, M. Gockeler, R. Horsley, D. Pleiter, P.E.L. Rakow, A.   Schafer, G. Schierholz and H. Stuben