I would suggest to have a look at http://www.kps.or.kr/home/kor/journal/library/downloadPdf.asp?articleuid=%7B5EFEB375-CF09-4A3C-8F1A-909464D47E0D%7D

It gives a good introduction. I suspect that the reason is that at high temperature the ac dielectric loss is dominated by the long range charge carrier transport but at low temperature both local and long range charge transport should be considered.

Hello Mr. Kumar

Which kind of "frequency dependence ac conductivity spectra " we are talking about? Which value is plotted against frequency?

But in general in impedance spectroscopy (ac conductivity spectroscopy) a "double plateau" could be found in several different plots and hints for two relaxation times. For perowskites it might be caused by the mixed (ionic and electronic) conduction mechanism of these materials. 

Hello Mr. Kumar,

which plot of frequency dependence ac conductivity spectra are we talking about? There are several representations of impedance spectroscopy ( ac conductivity spectroscopy). And which temperature range and frequency range are we talking about?

In general two plateaus in (for example) double logarithmic plot of Z' against ny point to two different relaxation times. In your case of a perowskite it might describes the mixed conductivity (ionic and electronic) of the material.

(I am little bit puzzled because I have added a similar answer one hour ago, but it is missing...)

edit: Now there are both answers... Excuse my double posting!

Dear Thomas, in some cases seemingly  the answers are edited or at least checked before they appear on the website. I observed it too.

@Sandeep: Gyorgys answer (nearest neighbor hopping, long range hopping) could be a possible explanation for two relaxation times as well. To discriminate conductivity mechanisms we will need more information about material, temperature, frequency, ...

I have attached a few publications, describing different (ionic, protonic, electronic) conductivity mechanisms in perowskites.  

@Gyorgy: Thanks for your answer and again excuse apossible confusion.

Article Oxide-Ionic and Protonic Conductors Based on Perovskite-Type...

Article Mixed ionic-electronic conductors: Effects of ceramic micros...

Article Mixed Electronic-Oxide Ionic Conductivity and Oxygen Permeat...

Article Mixed Ionic–Electronic Conductivity of La1–xSrxCo1–yFeyO3 Pe...