Negative Differential Conductivity in Gases: The "True Origin"

In a recent article [Yamamoto and Ikuta: J. Phys. Soc. Jpn. 68 (1999) 2602] state that the true origin of negative differential conductivity (NDC) has never been shown, in apparent contradiction of claims made in earlier papers [Z. Lj et al.: Aust. J. Phys. 37 (1984) 23: R. E. Robson: ibid. 37 (1984) 35]. In the present paper, we argue that the true origin of NDC has indeed been explained satisfactorily, and present a comprehensive range of calculations for a number of cases, demonstrating the effectiveness of momentum transfer theory [Hot electron transport in semiconductors (Springer, Berlin, 1985) p. 82] for understanding NDC from both a qualitative and quantitative perspective.