  The $\mathbf k$-dependent electronic momentum relaxation rate due to Umklapp
scattering from antiferromagnetic spin fluctuations is studied within a
renormalized mean-field approach to an extended $t-J$ model appropriate to
YBa$_2$Cu$_3$O$_{7-x}$ and other cuprates. Transport coefficients are
calculated in a relaxation time approximation. We compare these results with
those obtained with the phenomenological assumption that all scattering
processes dissipate momentum. We show that the latter, which violates momentum
conservation, leads to quite different magnitudes and temperature dependences
of resistivities and Hall coefficients.
