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 teaspoon of bacon grease from the pan. Add scallions to the grease and saute until soft, but not brown. Add bacon.
Drain potatoes and return to the pot. Mash potatoes with milk, sour cream, salt and pepper. Stir in the bacon and scallions. Reheat if necessary before serving.