Suppression of the Z-graph contributions in (e, e') at high momentum transfer

At high momentum transfer q > 1GeV, the longitudinal response in quasi-elastic electron scattering is well reproduced by the free Fermi gas model. This suggests the suppression of the Z-graph contributions in elementary electron- nucleon scattering in nuclear medium. Therefore, we develop the relativistic model in which those contributions are partially suppressed. The suppression redefines the relativistic effective mass and introduces the effective charge of a nuclear nucleon scattering the electrons. These quantities reproduce both the position and height of the quasi-elastic peak. The missing charge problem is also naturally explained.