  We studied mesoscopic Hubbard rings with impurities using the renormalization
group (RG) technique and the exact diagonalizations. The exact diagonalization
calculations showed that the charge stiffness normalized by the value of clean
system has a peak as a function of electron-electron interactions. Previous RG
analysis has succeeded to show the enhancement of the stiffness at weak
interactions, but does not show the peak behavior. We derived RG equations with
$4k_F$ impurity scattering term, which was ignored in the previous studies, and
reproduced the overall behavior of the exact diagonalization results.
