Drag GameObject with Mouse

I want to be able to drag a game object (a cube, for instance) in 3D space with a mouse (by left click and drag). However, instead of moving the cube up, down, left and right (in X-Y plane), I want to be able to move it forward, backward, left and right (in X-Z plane). The camera is perspective and slightly tilted down to look at the cube.

This is the script I am using now which works only approximate to an effect that I want:

if(Input.GetMouseButton(0)) {
	var pos : Vector3 = Input.mousePosition;
	pos.z = 10.0f;
	var pos2 : Vector3 = Camera.main.ScreenToWorldPoint(pos);
	pos2 = Vector3(pos2.x, m_character.transform.position.y, pos2.y);
	m_character.transform.position = pos2;

When you run this script, the cube moves with a slight offset from where the mouse actually seems to be pointing at. What I am looking for is a drag that accurately follows the mouse pointer, no matter where I click on the screen.

I am fairly new to Unity. So any sort of help will be greatly appreciated!

Thanks much!

Seriously guys, any one with any clue or idea, will be of great help to me!