It's a posthumous coin of Constantine I (Roman Emperor) - have a look here (search for quadriga to see similar coins):
http://www.wildwinds.com/coins/ric/constantine/t.html
I think it's from Constantinople - the mintmark in the exergue on the reverse has an O in it anyway - so it might be "Constantinople RIC VIII 37" or similar (see on the Wildwinds page).
Exactly which coin it is depends on the mint it's from.
ATB,
Aidan.