Quad to rotate towards camera on y axis only

I want to make a simple billboarding script that makes a quad follow the camera, but only limit it to the y axis. Currently I am using:

transform.rotation = Camera.main.transform.rotation;

However, this rotates the object on the x and z axes as well. I simply want to constrain it to the y axis but i have encountered some errors when trying to add the y axis.
I tried using LookAt but the object simply faces the camera away.
Any one who can help me achieve what i want?

I used this C# script… add it to the object you want, to follow the camera.

using UnityEngine;

public class Billboard : MonoBehaviour
	void Update() 
		// look at camera...
		transform.LookAt(Camera.main.transform.position, -Vector3.up);
		// then lock rotation to Y axis only...
		transform.localEulerAngles = new Vector3(0,transform.localEulerAngles.y,0);

crazy late but , i hate it when i stumble on posts that don’t have the answer I’m looking for. should work with whatever up axis you want, and stays in world space.

Vector3 upAxis = Vector3.up;
transform.rotation = Quaternion.LookRotation(Vector3.Cross(upAxis,  Vector3.Cross(upAxis, Camera.main.transform.forward)), upAxis);