The need for fast electronic devices working under extreme conditions, particularly at high temperature and high voltage, led researchers to investigate the use of films based on diamond(1-3), graphitic carbon(4), amorphous carbon(5) and other carbon nanostructures(6). In parallel, a different class of materials including disordered organic(7,8) and inorganic(9-11) materials has been studied, particularly for fast switching and large-area inexpensive electronics based on quantum transport(9,12). However, fast-switching devices of amorphous semiconductors based on negative differential resistance or resonant tunnelling has not been achieved so far(13,14). Here, we show negative differential resistance peaks, quantized conductance and bias-induced switching with a high-frequency response from amorphous-carbon quantum-well structures. We also demonstrate sufficiently large values for the phase-coherence length and delocalized conduction in these band-modulated low-dimensional disordered carbon structures, which could lead to a new generation of unusual fast-switching devices.