We have observed a series of sharp peaks in the low-temperature I(V) characteristics of a gated 1 μm×1 μm GaAs/(AlGa)As resonant tunneling diode, in which the gate is used to reduce the effective cross-sectional area from 0.7 to <0.1 μ${\mathrm{m}}^2$. These peaks, which occur at voltages well below the calculated resonant threshold, show a weak dependence on temperature, magnetic field, and cross-sectional area. We argue that this subthreshold structure is due to an inhomogeneity in the device, which gives rise to a localized preferential current path, and we deduce that the spatial extent of the inhomogeneity is approximately 25 nm. The likely origin of the inhomogeneity is a background donor impurity in the quantum well.

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