In a small saucepan heat sugar and 1/4 cup poire William over moderate heat until sugar is dissolved, about 5 minutes. Remove pan from heat and cool syrup. Spoon 1 1/2 tablespoons syrup into each of 4 Champagne glasses and add 1 teaspoon poire William to each glass. Add 2/3 cup Champagne to each glass and stir well. Garnish with pear slices.