Alcantara is the name of a town in Spain near Portugal, the name itself is derived from a bridge which has been standing there for centuries. The Bridge is the origin of our name, in arabic Al-Kantara is "the bridge". Hence the knight who originally guarded that bridge also adopted the name (as far as I can gather). Originally the name was De Alcantara (Of Alcantara) but has been shortened since. Alcantara is also one of the four military orders of Spain, originally one of the groups involved in the crusades.
Our family lives in Gibraltar, we moved here from Lisbon in the 1800's and are tracking down family members - so far we've discovered a James Alcantara in the States or Canada (I'll have to ask my dad about that one.)
By the way, the bridge still stands and is HUUUUUUUUGE.:)