Do you mean CDMA only? Not aware of an LTE only mode. I only see "LTE/CDMA" or just "CDMA only" under Network Mode (Menu, Settings, Wireless & network, Mobile networks).
You can try putting it on CDMA only for a while and see how it impacts battery life. But, from my experience, I think it has more to do with whether the phone has to search for signal and how much it switches in and out of data modes. I always leave it on the default LTE/CDMA and since LTE went live back in Oct. battery life has actually increased as the LTE signal is a lot stronger (at home and work and everwhere in between) so that it rarely if ever drops out of LTE. I've also noticed that with LTE, I get things done faster, thus the phone goes back to standby quicker than if I were using 3G. Anyway, your mileage may vary.