A new device based on side-gated wires demonstrates that the side gate wire technique can be successfully implemented as a novel selective depletion scheme for vertical tunneling devices. Resonant tunneling between 1D states was achieved by an additional lateral confinement generated by a central gate. From model considerations assuming a parabolic confinement, the tuning range of the subband energy was estimated to Lie between 0 meV and 5-6 meV. The transmittance of strongly coupled superlattices at different superlattice bias conditions is measured by varying the energy of the injected hot electron beam. The onset of scattering-induced miniband transport is clearly evident and the transition between coherent and incoherent electron transport in superlattices is observed for the first time. A coherence length of 150 nm and a scattering time of 1 ps is determined. The experimental result is in good agreement to a fully three-dimensional calculation including interface roughness with typical island sizes of 10 nm. This clearly demonstrates that interface roughness scattering Limits the coherence length of ballistic electrons in the superlattice.