An Android 4.4.2 KitKat firmware for the AT&T Galaxy S4 was leaked yesterday, but as we later found out, it was accompanied by a KitKat firmware for the carrier's Galaxy Note 3 as well. Carrying build number N900AUCECMLG, the Note 3 firmware has a build date of January 4, and seems to have most of the KitKat features implemented and ready to be used. However, the leaked firmware includes new bootloaders, as is the case with the S4's firmware, so you'll have to keep in mind that you might not be able to return to Android 4.3 if you try it out on your Galaxy Note 3.
Hit the source link for more details on the firmware and to download it (there's a one-click installer available for quick flashing.) As always, be sure to take the necessary backups before proceeding, since leaked firmware often have a tendency to wipe all data.