The measurement frequency ranged from 0.1 to 1000 kHz, and the voltage was 100 mV. We employed the 4-probe method. The resistance values were obtained from the Cole-Cole plots (see Appendix), and the conductivity was calculated based on the following equation , where the uniform electrode area is A [cm 2 ], the distance between electrodes is L [cm], the resistance is R [Ω], and the conductivity is σ [Scm -1 ].