Using optical techniques, we have demonstrated that the quantum coherence of electron states in GaAs/GaAlAs superlattices under electric fields increases drastically as the period, $D$, and the electric field, $\cal{E}$, decrease, reaching at least ten periods at zero field when . The electric field localizes the states, breaking the superlattice minibands into Stark ladders, and producing a transition from three- to quasi-two-dimensional excitons. These effects are manifested, respectively, by the appearance of oscillations in the absorption coefficient, which are periodic both in energy and in $\cal{E}^{-1}$, and by a sharp increase in the intrawell exciton binding energy.