I don't think it's right to just leave her hanging (that's so inconsiderate and you said she's a nice girl right?). Please don't hurt her like that. What about calling her when you know she's not home (like during work hours) and leave a nice message. Tell her you really liked her and had a good time but that you're just not interested in continuing the relationship. Wish her the best and say goodbye. Seems like the honorable thing to me.