CRJ900 flight crew receives apparently false brake temperature warning upon beginning descent from FL330.

Narrative

As we were beginning our descent from FL330 into ZZZ; the brake overheat warning illuminated and the EICAS displayed a left outboard brake temperature of 20 degrees. We performed the memory items of slowing to 220 KTS and extending the landing gear. As we were slowing down; the left outboard brake temperature indication was erratic; but gradually decreasing. The other 3 brake temperature indicators were all 00. Both the First Officer and I have seen this situation before. In my case; it was at FL370 and I learned quickly then that the plane isn't able to maintain a high altitude with the gear down. The ATC Controller; in my first experience with this; was completely unhelpful. When I told her that we were unable to maintain altitude and airspeed; I wasted no time in declaring an emergency to avoid an argument with ATC. With the emergency declared and memory items finished; we completed the emergency checklist to reset the BTMS overheat system. The reset was successfully; so we canceled the emergency and proceeded normally into ZZZ. Once on the ground; Maintenance decided to defer the left-hand outboard brake temperature indicator.