In the present paper it is pointed out that the frictional forces and the energy transfer rate appearing in the balance equations for the hot-electron transport problem all depend explicitly on the electric field. Their expressions are rederived with the electric field dependent corrections to O(E') included. These expressions are identical with those of Lei-Ting except that in the latter the electron energy appearing in the delta function and the distribution function are replaced by come sponding new-defined field dependent ones. The significance of such substitutions for obtaining the correct result is discussed in detail.

In view of the recent disputes of Huang and Wu and Lei about Lei's balance equation theory for an arbitrary energy band, the balance equations for hot-electron transport in an arbitrary energy band are reformulated. In the new formulation the adoption of the effective Hamiltonian method is avoided and all calculations are carried out in laboratory coordinates. In the mean time the crystal momentum k is carefully distinguished from the physical momentum. By introducing a correct equilibrium density matrix of the drifting electron gas in place of that used by Lei, the balance equations together with the distribution function can be derived directly with no need to distinguish the degree of freedom of the center of mass from the relative degree of freedom of the electrons, which is in our opinion neither possible nor necessary at least for a nonparabolic energy band. Some problems associated with the foundation of the balance equation theory are also examined. The result shows that the comment to Lei's theory given by Huang and Wu is correct and to the point. Nevertheless, contrary to their assertion that this theory is inapplicable to the nonparabolic band transport, Lei's balance equations, after a few, however, important corrections, are still applicable to transport for nonparabolic energy bands. I ) 96 Jingzhai Road, Anhui, Hefei 230026, People's Republic of China. 9 nhvcica (h\ 197/1

In this letter, we present an analysis to understand why the McMillan-like Tc formula: , proposed in a previous paper by the authors, applies to crystalline superconductors satisfactorily but less satisfactorily to non-crystalline ones.

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