In a large pot, cover potatoes and garlic with lightly salted water. Boil until potatoes are fork tender. Fry bacon in skillet until crisp; drain on paper towels. Pour out all but 1 tsp bacon grease from pan. Add scallions to grease, saute until soft, but not brown. Add bacon. Drain potatoes and return to pot. Mash potatoes with milk, sour cream, salt, and pepper. Stir in bacon and scallions. Reheat, if necessary, before serving.