The proposed sequential tunneling model predicts that the negative differential resistance in a double‐barrier diode (DBD) can exist independent of a resonant Fabry–Perot effect. The proof is based on the existence of quasi‐two‐dimensional states for electrons in the quantum well. However, we find that existence of the quasi‐two‐dimensional states depends on a Fabry–Perot resonance. Therefore, if coherence of electrons in the quantum well is completely lost, then the quasi‐two‐dimensional states will also disappear. We find that the damped Fabry–Perot model can provide a unified formula for electron transport in the DBD from pure coherent tunneling to pure incoherent tunneling. In the latter case, the negative differential resistance disappears.