If you had traveled the wrong way down the one way street for a nano-second you would be guilty of what you are accused of. The cop's driving record provides you no excuse for your mistake.
You need to decide how badly you want to avoid getting convicted for this offense. This is a moving violation which will result in a point or two added to your DMV record. However, you should be given the option of keeping your record clean by taking traffic school if you have not been enrolled in one recently. If you cannot take traffic school, and you already have points on your record, additional points might result in license suspension.
If you really want to fight this, your chances of success would be increased greatly if you got a lawyer. Lawyers familiar with traffic offenses, if they are diligent in what they do, know a few tricks in how to get them dismissed, even in such straightforward and seemingly hopeless situations as yours. If you want to do it yourself, I recommend you purchase a Nolo Press book on How To Beat a Traffic Ticket in California.
Feel free to contact my office is you want additional non-binding consultation.
Jacek W. Lentz, Esq.