  We discuss the convergence of the expansion of the nuclear electroweak
current in powers of |k|/M, where M is the nucleon mass and k denotes either
the momentum transfer or the momentum of the struck nucleon. We have computed
the electron and neutrino scattering cross sections off uniform nuclear matter
at equilibrium density using correlated wave functions and the cluster
expansion formalism. The results of our work suggest that the proposed approach
provides accurate estimates of the measured electron scattering cross sections.
On the other hand, the description of the current based on the widely used
leading-order approximation does not appear to be adequate, even at momentum
transfer as low as 300 MeV.
