Resonant tunneling characteristics originating from subband mixing at the emitter-barrier interface were studied in double-barrier diodes with a restricted lateral dimension. The current vs. voltage characteristics exhibited a series of peaks spaced by the voltages corresponding to subband splittings in the double-barrier region when a large number of emitter subbands contributed to the tunneling current. These characteristics exhibited peaks spaced by approximately twice the voltages corresponding to the subband splittings in the double-barrier region when the laterally confined lowest emitter subband mainly contributed to the tunneling current. These differences were well explained by the calculation of tunneling of electrons from two-dimensional subbands in the emitter to one-dimensional subbands in the double-barrier region with subband mixing taken into account. Application of a magnetic field parallel to the tunneling direction also revealed the subband mixing effects on the tunneling current. The observed field dependence of the current peaks compared well to tunneling generated by mixing electromagnetic subbands at the emitter-barrier interface.