Plasma and Lasers are completely different. Plasma boils through armor while Lasers go right through it. A direct hit from an AT-AT's main guns at a scarabs leg would most likely rip it off, or disable it beyond repair. Plus, two feet of armor that is way more advanced then the covenant's armor, I think a scarab's beam would not pierce the armor unless it was focused at the legs for at least 4 seconds.
AT-AT: Firepower, Range, Armor, Accuracy
Scarab: Agility, Mobility, Versatility
Unless it was ambushed, the AT-AT would win.

I want a sheep for Christmas...
I want a goat too. I can breed them and make a Soat. (Or a Geep)
Go Blue!!!