  We systematically investigated the temperature behaviors of the electrical
conductivity and Hall coefficient of two series of amorphous indium gallium
zinc oxides (a-IGZO) films prepared by rf sputtering method. The two series of
films are $\sim$700\,nm and $\sim$25\,nm thick, respectively. For each film,
the conductivity increases with decreasing temperature from 300\,K to $T_{\rm
max}$, where $T_{\rm max}$ is the temperature at which the conductivity reaches
its maximum. Below $T_{\rm max}$, the conductivity decreases with decreasing
temperature. Both the conductivity and Hall coefficient vary linearly with $\ln
T$ at low temperature regime. The $\ln T$ behaviors of conductivity and Hall
coefficient cannot be explained by the traditional electron-electron
interaction theory, but can be quantitatively described by the current
electron-electron theory due to the presence of granularity. Combining with the
scanning electron microscopy images of the films, we propose that the
boundaries between the neighboring a-IGZO particles could make the film
inhomogeneous and play an important role in the electron transport processes.
