My advice is to take a weekend and figure out if you still really want to do a PhD: Why did you want to pursue your field of study? What do you love about it? Why did you want to go for a PhD? What did you want to do with it? Do you still feel the same way?
After thinking about it generically, then think about your experience in your department and why you chose it in the first place. Is it still a good fit for you? If not, is there a way to fix that?
If after considering all of this, you still feel like it's not that right choice, there is absolutely nothing wrong with that. I'm really trying to emphasize that you're not quitting. You're just doing what will make you happy.
Good luck with everything!!!!