By using the ultrahigh vacuum plasma enhanced chemical vapor deposition system to prepare nc-Si:H films with high conductivity, the experimental results show that the conductivity of nc-Si:H films increases with decreasing the mean grain size of films. Hence, there exists a small size effect on the conduction process. Based on the experimental data, we used the effective-medium theory to calculate the partial conductivity&sgr;cof crystallites and&sgr;iof the interface conductivity, respectively. Otherwise, we found that there existed two structure phase change point results from the effective-medium theory calculated for the materials of silicon films. The results suggest that the high conductivity of nc-Si:H films results mainly from the crystallites, and moreover, the interface region may serve as insulator layers. Thus, we may consider that the crystallites in nc-Si:H films act as quantum dots. In this paper, we present a heteroquantum dot tunneling model to discuss the transport process for the nc-Si:H films. Our calculated results agree very well with the experimental conductivity data for nc-Si:H films. ©1997 American Institute of Physics.