  Incommensurate lattice fluctuations are present in the beta_L phase (T_c =
1.5 K) of ET_2I_3 (where ET is BEDT-TTF -
bis(ethylenedithio)tetrathiafulvalene) but are absent in the beta_H phase (T_c
= 7 K). We propose that the disorder in the conformational degrees of freedom
of the terminal ethylene groups of the ET molecules, which is required to
stabilise the lattice fluctuations, increases the quasiparticle scattering rate
and that this leads to the observed difference in the superconducting critical
temperatures, T_c, of the two phases. We calculate the dependence of T_c on the
interlayer residual resistivity. Our theory has no free parameters. Our
predictions are shown to be consistent with experiment. We describe experiments
to conclusively test our hypothesis.
