void OnMouseDown()
{
     screenPoint = Camera.main.WorldToScreenPoint(scanPos);

     offset = scanPos - Camera.main.ScreenToWorldPoint(
         new Vector3(Input.mousePosition.x, Input.mousePosition.y, screenPoint.z));
}

void OnMouseDrag()
{
     Vector3 curScreenPoint = new Vector3(Input.mousePosition.x, Input.mousePosition.y, screenPoint.z);

     Vector3 curPosition = Camera.main.ScreenToWorldPoint(curScreenPoint) + offset;
     transform.position = curPosition;
}